Question

In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to :

• A. Prevent interference by phosphate ions
• B. Decrease concentration of \( \text{OH}^- \) ions
• C. Increase concentration of \( \text{Cl}^- \) ions
• D. Increase concentration of \( \text{NH}_4^+ \) ions

Answer 1

The Correct Option is B

Role of Ammonium Chloride in Precipitation of Group III Cations:

In qualitative analysis, group III cations (such as \( \text{Fe}^{3+}, \text{Cr}^{3+}, \text{Al}^{3+} \)) are precipitated as hydroxides by adding ammonium hydroxide \( \text{NH}_4\text{OH} \).

Effect of Adding Ammonium Chloride:

Ammonium chloride \( \text{NH}_4\text{Cl} \) is added before ammonium hydroxide to control the concentration of \( \text{OH}^- \) ions. This is achieved through the common ion effect:

\[ \text{NH}_4\text{OH} \leftrightarrow \text{NH}_4^+ + \text{OH}^- \]

Adding \( \text{NH}_4\text{Cl} \) increases the concentration of \( \text{NH}_4^+ \) ions, which shifts the equilibrium to the left, decreasing the concentration of \( \text{OH}^- \) ions.

Importance of Controlled \( \text{OH}^- \) Concentration:

By reducing \( \text{OH}^- \) concentration, we avoid the formation of precipitates from cations of higher groups (such as Group IV and V cations), ensuring selective precipitation of Group III cations only.

Conclusion:

The addition of ammonium chloride decreases the concentration of \( \text{OH}^- \) ions through the common ion effect, which corresponds to Option (2).

Answer 2

Step 1: Context of Qualitative Analysis – Iron Group (Group III) Precipitation.
In qualitative inorganic analysis, the iron group (Group III) cations such as \( \text{Fe}^{3+} \), \( \text{Al}^{3+} \), and \( \text{Cr}^{3+} \) are separated by precipitating as their hydroxides.

Step 2: Role of Ammonium Hydroxide (\( \text{NH}_4\text{OH} \)) in Precipitation.
Adding ammonium hydroxide provides \( \text{OH}^- \) ions which react with metal ions to form insoluble metal hydroxides, thus precipitating the iron group cations.

Step 3: Why Ammonium Chloride (\( \text{NH}_4\text{Cl} \)) is Added Before Ammonium Hydroxide.
- Ammonium chloride dissociates in solution to give ammonium ions (\( \text{NH}_4^+ \)) and chloride ions (\( \text{Cl}^- \)).
- The presence of \( \text{NH}_4^+ \) ions shifts the equilibrium:
\[ \text{NH}_4\text{OH} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \] towards the left due to the common ion effect.
- This results in a decreased concentration of free \( \text{OH}^- \) ions in solution.

Step 4: Purpose of Controlling \( \text{OH}^- \) Concentration.
- If \( \text{OH}^- \) concentration is not controlled and is too high, cations from other groups (like Group IV and V) may also precipitate.
- Reducing \( \text{OH}^- \) concentration ensures selective precipitation of iron group hydroxides only, avoiding contamination with other metal hydroxides.

Step 5: Summary and Conclusion.
Therefore, ammonium chloride is added before ammonium hydroxide in the precipitation of the iron group (III) in qualitative analysis to decrease the concentration of hydroxide ions (\( \text{OH}^- \)) in the solution. This helps in selective precipitation of only the iron group cations.

Final Answer:
\[ \boxed{ \text{Decrease concentration of } \text{OH}^- \text{ ions} } \]